The invention relates to an improved method of and to an improved apparatus for making flexible laminates which contain substrates of leather. The invention further relates to flexible laminates which can be produced in accordance with the improved method by resorting to the improved apparatus. More particularly, the invention relates to improvements in a method of and in an apparatus for making a flexible laminate wherein at least one side of a substrate of leather is provided with a coating.
Leather is a preferred material for the making of a variety of useful and/or decorative products which can be put to use under a host of different circumstances. It is often necessary to subject hides or skins to extensive preliminary treatment (including tanning, dressing and others) before the thus obtained leather is in a satisfactory state for conversion into belts, bags, articles of footwear and/or many other products. For example, conversion of rawhides into calfskins or cowhides necessitates extensive treatment in order to ensure that the product will exhibit the desired flexibility and that its appearance will be acceptable for a particular purpose. Problems arise when large parts of treated skins must be discarded because they are damaged as a result of injury to the animals prior to or during killing. For example, pigskin often exhibits pronounced scars which are characteristic of pigskins or are the consequence of fights between live animals. Irrespective of their causes, such scars constitute blemishes which warrant discarding of corresponding portions of the skins.
The surfaces of skivers (i.e., thin soft leathers which are or can be made of the grain side of a split sheepskin) often do not resemble the surface of a piece of leather. This applies particularly for the grain or unevennesses in the surfaces of skivers and is the primary reason that skivers are considered second-class goods which are not used for the making of high-quality expensive leather goods. Therefore, workers in a tanning or other skin processing (finishing or dressing) plant must take care to avoid unnecessary expensive processing of skins or portions of skins whose appearance (particularly due to grain and/or other unevennesses) is unsatisfactory for the intended use of the leather. Accordingly, there exists an urgent need to provide a method of and an apparatus for salvaging skins and/or skin portions which must be discarded or used for the making of cheap products because their appearance is not satisfactory for the making of expensive leather goods.
U.S. Pat. No. 3,930,921 granted Jan. 6, 1976 to Connett for "Method of finishing leather" discloses a method which involves the transfer of a finish coating including an outer stratum of a thermoplastic acrylic polymer from a release layer bearing the finish coating to the leather. The leather and the release layer are pressed into a sandwich-like assembly with the concurrent application of sufficient heat and pressure to transfer substantially the entire finish coating to the surfaces of the leather and the backing layer juxtaposed therewith. The preformed finish can be directly applied to leather without prior treatment with adhesives or the use of so-called tie coats or other concurrently employed wet processing. The finish coating incorporates at least two distinct strata the first of which provides the base coat of the finish on the leather and is made up of thermoplastic. The second stratum forms the top coat of the leather finish and comprises a hard, non-tacky nitrocellulose lacquer. The thickness of the base coat is several times the thickness of the top coat to thus impart sufficient flexibility to the finished leather. The advantages of the method of Connett are that it can be carried out efficiently and economically, that it requires a single pass and that it provides more uniformly finished leathers than can be achieved in accordance with prior methods. Moreover, the finish coating can be transferred in the dry state without pretreatment with adhesives, tie coats and the like. Still further, the patentee emphasizes that the continuous finish film or coating produced in accordance with his method does not darken open scars or other blemishes in the leather surfaces but tends to level the color of differing portions of the leathers, such as scratches, scars or the like, more uniformly than conventional leather finishing processes.
Connett fails to disclose a method which renders it possible to fill and smooth the scratched and/or other damaged portions of leathers. The reason is that the patentee employs an adhesive acrylic polymer coating which constitutes the thermoplastic coating, and that the polymer is cross-linked. Thus, the coating which is used by the patentee does not flow but is merely applied in the form of a layer having a selected thickness to form one layer of a sandwich. Another drawback of the patented method is that Connett must operate at elevated temperatures in the range of 150.degree.-200.degree. C. which are necessary in connection with the making of sandwich-like structures having a layer consisting of a cross-linked polymer. On the other hand, treatment at such elevated temperatures will adversely affect certain important basic characteristics of leather, i.e., leather will become hard and brittle. The only possible mode of avoiding excessive damage to leather is to employ a relatively thick cross-linked acrylate layer which has a constant thickness and acts as a thermal insulator between the substrate of leather and the pressure applying tool.
The application of relatively thick layers to a substrate of leather is undesirable on the additional ground that this prevents the makers of such products from calling them "leather". For example, the Common Market countries adhere to regulations which permit the use of the term "leather" only if the product bears a plastic layer having a thickness less than 0.05 inch. The product of Connett employs a thermoplastic layer having a thickness of 0.07 inch plus a finish layer which causes the ultimate product to assume a thickness in excess of that contemplated for leather products by the relevant authorities of the Common Market.
U.S. Pat. No. 4,215,170 granted Jul. 29, 1980 to Oliva for "Metallization process" discloses the application of varnish to a substrate which can consist of leather. The patentee does not propose to fill in eventual scars and/or other defects of the substrate. The purpose of the method, the same as that of the method of Connett, is to create the impression of a smooth exposed surface on a substrate.